[mesa-users] Chemical diffusion and white dwarf atmospheres

[Bill Paxton]

Hi Chris,

Excellent questions!  diffusion with gamma > 1 is on the list, but isn't currently be worked on (at least not be me!).
Lars may have something to add since that's a topic of interest for him.

Concerning atmosphere effects -- the default behavior of mesa is to place the outer edge of the star model at the photosphere.
The atm module then deals with tau smaller than that and returns a surface pressure at that boundary.
The star module deals with tau > photosphere by means of the usual eos and opacities.
The boundary condition at the surface is that atm and star agree on T and P for the outer boundary of the star model.

If that's is your mode of operation, then at tau abound 25, you are below the photosphere, so mesa star is calcuating things.
Then it is a question of whether the eos and kap modules are going to produce good results for that part of the star.

For cases where you really need a non-grey atmosphere (such as brown dwarfs or giant planets), then we move
the outer boundary for mesa star to a larger optical depth and use a special atm option.  An example is the
 comparison in the paper with the Baraffe et al results.  For that we put the outer edge at tau=100 and use
the atm option to use the COND tables.  However those tables for great for brown dwarfs and planets, but
they stop at log(g) << what you need.

As far as other things not in mesa yet that might be relevant, I'm not the one to comment, but I'll certainly
be interested in what you learn!!!

Cheers,
Bill



On Feb 22, 2011, at 10:03 AM, Chris Richardson wrote:

> Hello all,
> 
> I don't have much experience with MESA but I've been exploring WD  
> cooling with it. I was curious if anyone has plans in the works to  
> incorporate chemical diffusion and gravitational settling where the  
> Coulomb coupling parameter is greater than unity.  Also, does MESA  
> implement non-ideal effects of gas EOS and chemical equilibrium into  
> atmospheric calculations at large gravities (say log(g) = 6.0 to 9.0)  
> at optical depths around tau = 25? What other treatments besides  
> crystallization (which is uncertain in all codes) currently hinder  
> MESA from reproducing accurate early and late time WD evolution? Any  
> insights would be greatly appreciated. Many thanks!
> 
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